1. Field
Exemplary embodiments of the present invention relate to light emitting devices having inorganic phosphors based on silicate compounds.
2. Discussion of the Background
Phosphors may be used in light sources, such as light emitting diodes (LEDs) that emit colored or white light. In an LED, phosphors, which may be used in combination with other luminophores, are used to convert ultraviolet or blue primary radiation emitted from an LED into longer wavelength secondary radiation, in particular, white light.
Although various phosphors having a high luminescent output, such as, cerium-doped yttrium aluminum garnets, europium-activated alkaline earth metal orthosilicates, and similarly doped nitrides having various compositions, have already been disclosed for these applications, further attempts have been made to develop improved materials for use in LEDs. Such development trends consist of, in particular, finding phosphors having improved temperature characteristics and having higher stability under a resulting radiation load, under the influence of atmospheric humidity, and under other environmental factors. Such luminophores may be required for the production of LED lamps having relatively high power consumption and improved lifetimes.
Europium-activated alkaline earth metal oxyorthosilicates of the general formula Sr3SiO5:Eu have been used for LEDs that emit colored or white light. Such phosphors are described, for example, in WO 2004/085570A1 and WO 2006/081803A1, and in various scientific publications, such as “Application of Strontium Silicate Yellow Phosphor for White Light-emitting Diodes” by Park, Joung-Kyu, et al., in Appl. Phys. Lett. 84 (2004), 1647-49, and “Photoluminescence properties of Eu2+-activated Sr3SiO5 phosphors” by Lee, Soon-Duk, et al., in J. Mater. Sci. 41 (2006), 3139-41, the entirety of which are hereby incorporated by reference in their entirety for all purposes, as if fully set forth herein.
The known luminophores emit light in the yellow to orange range of the visible spectrum and are distinguished by high luminescent efficiency and extremely low thermal quenching, up to a temperature of 250° C. In this respect, these luminophores are substantially superior to the orthosilicates, which also emit light in the range of 580 and 610 nm, as an orange component in phosphor mixtures for warm white LEDs and, owing to their advantageous properties and substantially lower production costs, may compete even with the red-emitting nitride phosphors that are increasingly favored for these applications.
Under certain conditions, however, LEDs including such phosphors may have a comparatively short lifetime. A possible cause of this disadvantageous behavior may be the relatively high moisture sensitivity of the europium-doped alkaline earth metal oxyorthosilicates. Due to such instability, industrial applicability of these luminophores may be limited in certain areas.